refactor fix private attribute warning

Author: vipulnsward

lib/thor/actions/file_manipulation.rb

@@ -294,8 +294,10 @@ def remove_file(path, config={})
     alias :remove_dir :remove_file
 
   attr_accessor :output_buffer
+  private :output_buffer, :output_buffer=
+
+  private
 
-  private :output_buffer
     def concat(string)
       @output_buffer.concat(string)
     end